The present invention is a reactor for the hydrogenation of chlorosilanes. The reactor employs a reaction chamber and a heating element formed from silicon carbide coated carbon fiber composite and employs silicon nitride to electrically insulate the heating element.
In a typical process for producing hyperpure semiconductor-grade silicon, trichlorosilane gas is reduced in the presence of hydrogen and deposited onto a heated element. A significant portion of the trichlorosilane gas fed to such a process is de-hydrogenated to form by-product tetrachlorosilane. It is desirable to convert this by-product tetrachlorosilane back into trichlorosilane which can be recycled to the deposition process.
Rogers, U.S. Pat. No. 3,933,985, issued Jan. 20, 1976, describes a process for converting tetrachlorosilane to trichlorosilane. The process involves passing hydrogen and silicon tetrachloride vapors through a reaction chamber held at a temperature of between 900.degree. C. and 1200.degree. C.
More recently, Weigert et al., U.S. Pat. No. 4,217,334, issued Aug. 12, 1980, described an improved process for converting tetrachlorosilane to trichlorosilane. The process involved reacting trichlorosilane with hydrogen at a temperature of 600.degree. C. to 1200.degree. C., with a tetrachlorosilane and hydrogen mixture having a molar composition between 1:1 and 1:50 in equilibrium with trichlorosilane and hydrogen chloride, and suddenly quenching the mixture to below 300.degree. C. The process described by Weigert et al. was conducted in a gas-tight tube constructed from carbon.
The use of carbon and carbon based materials, such as graphite, for construction of the reaction chamber for the process described by Rogers and by Weigert et al. suffers numerous shortcomings. For example, pressure differentials, high temperatures, and rapid temperature changes experienced in the reactor causes extreme thermal stress on the reactor components often resulting in the need to shut the reactor down for repairs. Furthermore, the carbon-based materials can react with the chlorosilane and hydrogen feed to form by-products, such as silicon carbide, methane, and carbon monoxide. These reactions not only cause deterioration of the reactor, but can also contribute to carbon and trace metal contamination of the desired hydrogenated chlorosilanes.
The present invention is a reactor which employs materials of construction designed to reduce these problems. The described carbon fiber composites provide high strength with good elasticity properties, thus providing greater resistance to pressure and thermal stress damage of the reactor. Furthermore, the coefficient of expansion of the carbon fiber composites can be tailored to closely match that of a silicon carbide coating. Therefore, thermal expansion of the carbon fiber composite is less likely to result in thermal fracture of a silicon carbide coating. The silicon carbide coating placed on the carbon fiber composite provides protection against the reductive processes which can cause deterioration of the reactor chamber and the heating element and contamination of the hydrogenated chlorosilanes.
Levin, U.S. Pat. No. 4,737,348, issued Apr. 12, 1988, describes a reactor in which hydrogen gas and tetrachlorosilane are reacted to form silicon at temperatures above about 1500.degree. C. The reactor has walls formed from carbon or graphite and a silicon carbide coating is formed in situ on the walls. Levin describes the silicon carbide coating as being highly resistant to chemical decomposition. Levin does not address the problem of thermal shock associated with high temperatures and with heating and cooling of the reactor nor the potential problem of fracturing of materials due to pressure differentials.
The present invention can also use silicon nitride to electrically insulate the heating element employed to heat the reaction chamber. In the present reactor for reacting chlorosilane and hydrogen, it is not feasible to completely seal the reaction chamber to prevent leakage of the reactants. Therefore, the material used to electrically insulate the heating element can be subjected to chlorosilane and hydrogen at elevated temperatures. The inventors have found silicon nitride to be an excellent electrical insulating material with minimum deterioration under typical process conditions. Witter et al., U.S. Pat. No. 4,710,260, Dec. 1, 1987, describes a process for the hydrogen decomposition of trichlorosilane on silicon nitride particles. The silicon nitride is not considered for use as an insulating material and is reported to deteriorate significantly under the described process conditions. Therefore, somewhat surprisingly the inventors have found that silicon nitride is a suitable electrical insulating material for use in a reactor for reacting hydrogen and chlorosilane and that it does not contribute significant contamination to the hydrogenated chlorosilanes.
Therefore, it is an objective of the present invention to provide an improved reactor for the reaction of hydrogen and chlorosilanes. The reactor employs a silicon carbide coated carbon fiber composite as the material of construction for the reaction chamber and for the heating element. This material provides excellent resistance to reductive processes and high tolerance to thermal shock. The reactor can also employ silicon nitride to electrically insulate the heating element. Silicon nitride provides excellent insulating characteristics and high resistance to chemical attack by reactants present in the reactor.